You'll have to judge what you need. E-Commerce can be anything from payment processing, storage facilities, ordering (and keeping track of) physical stock, shipment, processing returns, customer service, payment fraud detection and many more. Or it can be just a membership site.
If you need an ecommerce solution (as it might have implemented quite a bit of the list above), you can always also integrate it with Liferay, so that Liferay provides the frontend and user interface, while everything else is contained in your existing (or new) ecommerce system. This is what some people have done
